The present invention relates to optical systems and elements, and more particularly to such systems and elements for obtaining uniform illumination.
Concentrated laser light that uniformly illuminates a target surface is required in many applications of the laser-matter interaction, particularly direct-drive laser fusion. In the configurations presently used for such applications, the laser light is concentrated onto the target by a lens. In order to achieve the desired intensity and to increase the illuminated area, one normally displaces the target from the best focus (far-field) of the lens to a position where the beam is still converging (near or quasi-near field). The difficulty of maintaining good beam uniformity on the target in this focal configuration is well known, particularly in cases where high powers are required. Amplitude nonuniformities in the laser beam at the lens will be imprinted on the target unless they are spatially small enough that diffraction smooths their effects before the beam reaches the target. These amplitude nonuniformities can be caused by damage or optical imperfections in the laser. In large high power lasers, which have many components, the cumulative effect of small phase aberrations introduced by each element (even those of high optical quality) can produce large intensity nonuniformities at the laser output. These can only be partially controlled at great expense by using ultra-high quality laser components, and using telescopes to reduce the transfer of phase aberrations into amplitude nonuniformities.
Several approaches have been suggested to alleviate the uniformity problem, none of which has proven practical. One approach is to overlap many laser beams onto a target, and rely on statistical smoothing. Unfortunately, the degree of smoothing increases only as the square-root of the number of beams. Laser-fusion may require intensity uniformities of a few percent. The hundreds or perhaps thousands of laser beams required to achieve such levels of uniformity by this approach would therefore be very expensive, and probably too complicated to be practical. Optical phase conjugation techniques have also been proposed for improving laser beam quality. These techniques are effective at enhancing the ability of aberrated lasers to produce a nearly diffraction-limited focal spot; however, even small deviations from perfect conjugation tend to produce large beam nonuniformities.
In the far field region (near focus) the nonuniformities present at the laser output tend to diffract out into a low intensity halo well beyond the main focal spot, and therefore pose a much less serious problem. However, this approach would have serious difficulty in achieving the desired focal spot diameter with normal high power lasers. For the case of light at near optical wavelengths (.about.1 .mu.m) and incident laser beams above 10 cm diameter, extraordinarily long focal length lenses (hundreds of meters) would be required to illuminate a millimeter section of a fusion pellet. This scheme is impractical because of the large focal lengths required, the accompanying high pointing accuracy required of the laser, and the large influence that phase nonuniformities across the laser beam would have on the focal spot diameter.
A technique has been recently proposed by Mima and Kato in ILE Progress Report on Inertial Fusion Program, No. 1, pp. 15-18 (May 1982) in which the laser beam is broken up by a random phase mask. In that proposal, however, the random phase relationship among the beamlets would remain fixed in time; i.e., the incident beam becomes aberrated, but not really incoherent. The focal interference pattern therefore persists throughout the pulse, and it invariably contains longer wavelength nonuniformities in the focal distribution that would be deleterious to the uniformity of the laser irradiance. Similar considerations apply to optical beam integrating devices that are designed to produce a "top hat" spatial profile such as U.S. Pat. No. 4,195,913 issued to D. Dourte et al.